High Pointe Commons
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|High Pointe Commons is a large project with 6 different
walls totaling more than 24,000 ft2 (2230 m2). The
largest wall is over 18 ft (5.48 m) tall and 710 ft (216.4 m) long.
Although large, a wall such as this is fairly straight forward to plan,
design and build. Walls, big or small, have the same questions to ask,
how tall does it need to be, how long should the geogrids be, are there
above or below, and are there surcharges above and what are
the soil conditions? Finding specific answers for these generic
questions would allow any engineer to design a wall just like this one.
This profile will discuss planning and designing of three particular
aspects, small slopes below, surcharges above and planning and
installing fence footings.
|High Pointe Commons is a large multi-faceted commercial
development with shops, restaurants and a larger up scale Sheraton Hotel
complex. The plan for a site as large as this requires the coordination
items such as storm pipes and manholes, traffic
flow patterns and many other things long before any construction can
begin. Retaining walls can play the essential role in making all of
these site development issues successful.
|A wall design engineer reviewing the site plans must not only
recognize and plan for the simple, accepted design issues such as
geogrid lengths and spacing, but also the more complex such as detention
basins and structures within the geogrid zone. A careful methodical
review of the site plans and a visit to the site, paying attention to a
Wall-Design Checklist (page 11 of AB Spec Book), will help minimize future unknowns in the
design and construction process. The main purpose of a wall design
checklist is to help the design engineer ask the right questions and to
make the design process run smoothly. Allan Block has a detailed
Wall-Design checklist that can be found in the Allan Block Spec Book.
Additionally, Allan Block has a
Construction and Inspection Check List
to assist the contractors and engineers with preconstruction meetings.
|Consider the 18 ft (5.48 m) tall by 710 ft (216.4 m) long wall shown
here. It was built with AB Classic block which has a nominal setback of
6 degrees. It has a flat top with a heavy traffic surcharge above and a
relatively short 2:1 slope below. When designing a wall with a slope
below, the engineer must always consider the stability of the slope.
Even a relatively short slope, such as this one, can have problems if
the wall is not built deep enough into the slope so the toe of the
wall does not become undermined or slip out. Depending on the soil type
and slope a
analysis may by necessary using a computer program
specifically designed to model slopes. For this discussion we will
discuss simple undermining. A typical amount of buried block for a wall
with no slope below would be roughly equal to 1 inch (25.4 mm) of buried
block for every 1 ft (0.3 m) of wall. So our example 18 ft (5.48 m) wall
would have about 18 inches (0.45 m) of buried block, but that would not
provide much resistance to undermining.
|When a slope is below a wall you should consider using a
5 ft (1.52 m) soil bench at the base of the wall. Reviewing the two
drawings shows that a 5 ft (1.52 m) bench provides a deeper wall rock
base and a much greater slip arc to resist undermining than does the
typical amount of buried block. Remember that with a 2:1 slope below and
a 5 ft (1.52 m) bench the wall would need 2.5 ft (0.76 m) of additional
height. This 2.5 ft (0.76 m) along with standard buried amount makes our
18 ft (5.48 m) exposed example wall closer to 23 ft (7.0 m) for design.
|A typical design section for a wall such as this one would be
similar to this section from
AB Walls Design Software.
The standard grid lengths would be around 60% of the total wall height and the top few
layers of grid, depending of the wall height, would be lengthened to 90%
of the total wall height due to the paved surfaces above. It is common
practice to lengthen top grids for three distinct conditions:
|At High Pointe Commons the majority of walls had either paved surfaces or slopes above them. AB Walls 2007 Design Software allows the engineer to model the wall section with elongated top grids under all surcharge conditions. With this flexibility, the user can attempt to model the design as close to as-built conditions as possible. The reason to lengthen the top grids when paved surfaces are above is to minimize the potential of future cracking in the retaining wall.|
|Cracked pavement can shorten the life of the pavement
but also the life of the retaining wall by allowing water to migrate
through the crack into the infill zone which could cause hydrostatic
pressure to build up behind the facing. An engineer should consider the
reinforced soil mass
and retained soil mass
as separate structures working independently. Although there is no relative movement
between the two zones, a defined construction joint remains because the
reinforced soil mass, formed by the Allan Block facing, the infill soil
and the geogrid working together as a single unit is built with a
tighter control on compaction. Whereas the retained soil is typically
unreinforced re-compacted site soil built with less controls. By
bridging the construction joint between the soil zones with extended
geogrid layers you disrupt any variations between them, such as
settlement, and minimize the potential of future cracking of the paved
surface and ultimately increasing the life of both the pavement and the retaining wall.
|For projects of any size, but especially for ones as large as High Pointe Commons, all responsible parties should convene at a preconstruction meeting to discuss and coordinate the construction and staging of the walls. From excavating, utility installation and material storage to railing installation and final grading everyone needs to coordinate their efforts to keep the project on schedule. Once the initial excavation is complete, construction of walls this large can be built efficiently using a separate base and wall crews. On long flat walls such as these, a base crew should start in the middle and work in both directions. Once the base course is complete they can move on to the next wall while the wall crew can start to build the body of the wall one course at a time. Having two crews running will greatly increase the speed and efficiently of the construction.|
|As you can see from this picture, the site engineer approved the use
of site soils as infill soils. It is important that the installer not
assume this is acceptable for every wall. The engineer must approve the
reuse of the excavated material and if they do, a careful staging plan
for the approved material should be implemented.
When reinstalling soil, a contractor must build the wall and backfill in one course increments. These maximum lifts of 8 inches (0.20 m) are important for proper compaction. Installers should verify with the engineer what level of compaction is required. A common value is 95% of Standard Proctor. Typically, a site this large would have an on site soils engineer to frequently test the compaction. Only light weight compaction equipment can be used within 3 ft (1 m) of the back of the wall facing and may require shorter lifts to meet the approved compaction level.
|This project called for over 2000 ft (610 m) of fencing or handrails
to be installed after wall construction. Once again, during the
preconstruction meeting the engineer, wall installer and railing
installer should coordinate their efforts. This small amount of extra
time and coordination can save immeasurable time and prevent potential
problems for the completed project. If the wall installer works with the
designer and railing installer, a depth and location of each post
footing can be worked into the wall construction. Going back after the
wall is complete to install the post would most likely require hand
digging as power hole augers can damage the geogrid to a point of
No matter what the size of your project, planning ahead and taking a methodical approach to design and construction is the key to building a quality structure with a lifetime of service.